1. Field of the Invention
The present invention relates to a traffic information providing device, a traffic information providing system, a traffic information transmission method and a traffic information request method capable of estimating and interpolating missing data of probe data (traffic information) in real time and providing comprehensive estimating/interpolating information to probe terminals.
2. Description of the Related Art
The purpose of a traffic information service is to inform drivers the congested places, to guide the drivers along an optimal route, and to estimate required time. Further, from a societal perspective, environmental load can be reduced and economic losses caused by road traffic congestion can be restricted by effectively utilizing limited route resources and keeping demand-and-supply balance.
When the traffic information service was firstly started, the service was limited to provide current traffic information for a specified road district. Today, the traffic information service is extended and diversified into various styles. The traffic information service is extended mainly into two directions. One is the extension in time which is achieved by providing predicted traffic information. By using the predicted traffic information, it becomes possible to select the optimal route and estimate the required time even for a long distance driving or a long-term delivery plan. The other one is the extension in space which is achieved by widening the area where the traffic information is provided by employing probe cars. Owing to the probe cars, even the traffic information of the routes (links) uncovered by existing road sensors can be collected and provided, so that the service can be provided effectively.
Herein the traffic information means travel time of the links, for example.
Similar to a VICS (Vehicle Information and Communication System), each probe car serves as a sensor, and therefore can collect the traffic information independently without depending on road infrastructure, and basically can provide traffic information of all routes. However, since the travel position of the probe car is random, and since the timing when the probe data (the data transmitted by the probe car) is transmitted to the probe data center are random, the quality of the information is quite different from the continuous data collected by the road sensors, namely, the data includes a large amount of missing data in time series and space series.
For example, if there are 100,000 probe cars in the whole Japan, then the probe data can be generated at a time density of around one per hour for each link. Compared to the data acquired by the road sensors, which acquire data at a time pitch of five minutes, the rate of missing data is up to 80% to 90%. In order to make use of the sparse probe data as a complementary information source to the data acquired by the road sensor, some kind of data interpolating technique is needed.
As a technique for interpolating the missing traffic information data of the links based on the traffic information data collected by the road sensors of the other links, there has been known an art in which, for example, traffic information of the links having missing data is estimated based on the traffic information of the upstream links and downstream links connected with the links to be interpolated, or based on the traffic information of the links parallel to the links to be interpolated (see Japanese Patent Laid-Open Publication No. Hei07-129893, paragraphs [0012] to [0038], FIG. 1 and the others).
Further, there also has been known an art in which the missing data of the traffic information of the links is estimated and interpolated by generating, based on the past probe data, a feature space which expresses the correlation among a plurality of links, and projecting current probe data to the feature space in accordance with its missing data. The merit of the method of estimating and interpolating the missing data by performing projection in feature space is that it is possible to use various correlated information contained in the past probe data to perform interpolation by decomposing the traffic information into components which are correlated among the plurality of links, calculating the interpolation data for each component, and combining the interpolation data for each components (see Japanese Patent Laid-Open Publication No. 2005-64767, paragraph [0007], FIG. 1, and “Realtime Interpolation Method for Probe Car System”, Masatoshi Kumagai et al., IPSJ SIG technical reports. ITS. Vol. 2005. No. 21 (20050310) pp. 67-73.
However, according to the art disclosed in the Japanese Patent Laid-Open Publication No. Hei07-129893, when the rate of the missing data of the probe data (the rate of missing data) becomes high, there are links whose probe data can not be acquired. For this reason, it is not possible to estimate the traffic information among the plurality of links based on such probe data. In other words, when the rate of missing data of the probe data becomes high (namely when the number of the links whose missing current traffic information is increased), there is strong possibility that all information of the links around the link to be interpolated is unknown, so that the interpolation based on the surrounding links is impossible. Further, even in the case where the missing data of the traffic information of a certain link can be estimated based on the surrounding links, when connection relation of the links is complicated, the precision of the estimation will become significantly low, so that the estimated information will significantly diverge from the current traffic information.
Though it will be possible to estimate the missing data of the link having high rate of missing data by using statistically processed past probe data, the statistically processed probe data reflects the past data after all, it does not necessarily reflect the current information.
On the other hand, according to the arts disclosed in the Japanese Patent Laid-Open Publication No. 2005-64767 and the “Intelligent Transport System”, the traffic information of the links (the links with missing current traffic information) for which no current probe data (current traffic information) is collected can be estimated and interpolated from the collected current probe data of the links other than the links to be interpolated based on previously calculated correlated traffic information among the links.
Further, since the estimation and interpolation is performed without discriminating the links having low degree of correlation from the links having high degree of correlation, interpolation calculation will be unstable for the links having low degree of correlation. As a result, the information (including the current information and the interpolated information) for a specified link is apt to be missed. Moreover, since the degree of correlation of links can not be determined unless from the result of the estimation and interpolation, the fact that the links having low degree of correlation can not be estimated and interpolated can not be known until the estimation and interpolation is performed, so that the cover ratio of the area where the traffic information is provided can not be efficiently widened.
Herein, the “connection relation of the links” means the actual connection of the links, and the “correlation of links” means the case where the traffic information of a certain link affects the traffic information of the other links. For example, when the traffic information (travel time, for example) of link A increases, if the traffic information the other link B also increases along with the link A, then we say that the traffic information of the link A is in correlation with the traffic information of the link B. If the traffic information of the link B increase/decrease in no correlation with the traffic information of the link A, then we say that the traffic information of the link A is not in correlation with the traffic information of the link B.
Hereinafter, the traffic information of links in/not in correlation with each other is referred to as “the links in/not in correlation with each other”, and the traffic information of links have high/low correlation with each other is referred to as “the links have high/low correlation with each other”.